The present invention relates to a method and apparatus for measurement of surface shape or topography of a finely processed matter or the like, by using a scanning electron microscope (hereinafter simply abbreviated to "SEM"). In particular, the present invention relates to a method and apparatus for measurement of surface topography using an SEM, which is made possible to perform high-accurate measurement by employing a surface topography measurement correcting method in which the result of measurement with respect to a sample on a sample table through the SEM can be corrected only by observing the same sample with the sample table inclined without the necessity of using any specific sample for correction.
Recently, attempts to reproduce surface topography based on the information of image concentration have been extensively carried on. For example, in Japanese Patent Unexamined Publication No. 62-6112 (Japanese Patent Application No. JP-A-60-144434 filed on July 3, 1985 by President of the University of Tokyo), there is disclosed a method for obtaining surface topography by using an SEM with four reflected electron detectors, in which the relation of reflected electron detection outputs and normal lines obtained by use of a reference sample in advance are tabulated, a normal line at each point of the surface of a sample is obtained on the basis of the tabulated relations, and the inclination of the sample surface is integrated on the basis of the information of normal line to obtain the cubic configuration of the surface. On the other hand, in Japanese Patent Unexamined Publication No. 56-150303 (Japanese Patent Application No. JP-A-55-53574 filed on April 24, 1980 by President of the University of Tokyo), Japanese Pat. No. 462,147 (Japanese Patent Publication No. 40-17999, Japanese Publication Application No. JP-A-38-43863 filed on September 13, 1963 by JEOL Ltd.), and Journal of Electron Microscopy vol. 34, No. 4, pp. 328-337, 1985, "Measurement of Surface Topography Using SEM with two Secondary Electron Detectors", Tadao SUGANUMA, there is a method in which two detectors for detecting reflected electrons or secondary electrons are arranged so as to be opposite to each other above a sample, so that the inclination at each point on a virtual line imagined along the sample surface and parallel to a direction connecting the two detectors is obtained on the basis of the experiential fact that the difference in value or in square value between the respective detection outputs obtained from the detectors is approximately proportional to the inclination of the sample surface in the direction connecting the two detectors, thereby obtaining the surface topography on the basis of the information of the inclination.
The method disclosed in the Japanese Patent Unexamined Publication No. 62-6112 has disadvantages in that a large time is required in operation of determining the normal-line direction through comparison between the values of the sample and reference sample, and in that, when the reference sample has a local crack or the like, an error may be caused in the reference signal to thereby affect the result of the measurement directly.
Further, there arises a problem in that the calculation expression of inclination is experiential and insufficient in exactness and, in particular, when an inclination component exists in a direction perpendicular to the direction connecting the detectors, the value of inclination becomes inaccurate.
Another method of measurement is discussed in an article by Tadao SUGANUMA, entitled "Measurement of Surface Topography Using SEM with Two Secondary Electron Detectors", Journal of Electron Microscopy, Vol. 34, No. 4, pp. 328-337, 1985.
In this method, two detectors are provided on an SEM, a difference between the two square values of image signals from the two detectors is obtained so as to obtain an inclination of each pixel on a line in a direction connecting the two detectors, that is, a differential value of the in a direction connecting the two detectors, and the thus obtained differential values with respect to the respective pixels are added successively from the end value so as to be integrated, thereby obtaining surface topography on the line.
In practical use, the calculation expression of the inclination of the sample surface is expressed as follows. EQU tan .theta.=k(A.sup.'2 -B.sup.'2)/(A'.sub.n +B'.sub.n).sup.2( 1)
In the expression (1), A' and B' represent the values of the output signals of the left and right detectors, and .theta. represents the inclination angle of the sample surface.
In the expression (1), the denominator serves to cancel a change in intensity of an incident electron beam, A'.sub.n and B'.sub.n represent the intensity of the output signals of the left and right detectors when an image of a flat portion of the sample is picked up, and k represents a proportion constant which is determined experimentally. In particular, the value of k is obtained from a correction sample formed of a 7 .mu.m-diameter latex resin ball coated with gold to prevent electrification. By use of the fact that the true value of surface inclination at each point of a sphere can be generally calculated from the relative position on the image of the sphere, the correction of measurement is carried out by comparing the inclination calculated from the image of the correction sample by the expression (1) with the inclination theoretically obtained with respect to the sphere.
In this method, however, it is necessary to prepare a correction sample and to carry out a predetermined correction process, whenever the proportion constant in the expression (1) is changed according to circumstances, such as a change in accelerating voltage, a change in working distance (the distance between an objective lens and a sample), a change in secondary electron pull-in voltage, or the like.
Further, in the case where the measurement condition of a correction sample is not perfectly in accordance with the measurement condition of a sample to be subject to stereo-measurement, there arises a problem that the stereo-measurement is lowered in accuracy.